A new study has concluded that the massive collection of phone data by the clandestine U.S. National Security Agency "has had no discernible impact" on preventing terrorism.

A Washington research group, the New America Foundation, said Monday it studied the investigations of 225 people linked in some way to terrorism in the United States since the deadly September 11th attacks and concluded NSA phone surveillance only played a key role in one instance.

The report said the only piece of NSA phone data that had a clear role in initiating an investigation involved a cab driver in San Diego, California, who was convicted of sending $8,500 to al-Qaida's Somali affiliate in 2007 and 2008.

The New America Foundation said NSA surveillance may have played a role in other investigations, but about 60 percent of the probes stemmed from traditional investigative methods, such as tips from from a family member or informant, or a report of suspicious activity.

The report's conclusion mirrors that of a White House-appointed review that concluded in December that the NSA's collection of millions of records of calls made by Americans "was not essential to preventing attacks."

The separate reports come as President Barack Obama plans to announce Friday whether he will curb NSA surveillance programs, including the phone data collection and monitoring of calls made by foreign leaders.

For several years, the NSA has accumulated records of the numbers called by Americans, the dates and lengths of the calls, but not their content, in an effort to stop new terrorist attacks. When this practice was first revealed last year, NSA director General Keith Alexander defended the data collection, saying it had helped prevent more than 50 potential terrorist attacks in 20 countries around the world.

The scope of the NSA's spying has become evident in recent months through leaks from former U.S. national security contractor Edward Snowden. The NSA says he stole 1.7 million documents while stationed at an NSA outpost on the Pacific island state of Hawaii. He is now living in asylum in Russia even as American authorities seek his extradition to stand trial in the United States on espionage charges.

Two U.S. judges have issued rulings in separate cases seeking to end the phone data collection. One upheld the program and the other said it likely violated the U.S. Constitution's prohibition against illegal searches.